In common rail injection systems a plurality of injectors (injection valves), typically all the injection valves of the corresponding internal combustion engine, are supplied with fuel via a common feed, the fuel being maintained under pressure by means of a common high-pressure pump. For this purpose the fuel can be supplied to the high-pressure pump from a tank by means of a fuel delivery pump. A pressure control valve or a flow control valve is typically disposed upstream of the high-pressure pump for the purpose of setting a pressure, referred to here as a rail pressure, in the common feed of the common rail injection system, i.e. between the high-pressure pump and the injection valves.
The amount of fuel injected into a cylinder during a single injection (injection quantity) is controlled via an injection valve (injector) assigned to said cylinder. However, a prerequisite for precise control of the injected fuel quantity is knowledge of the rail pressure in the common rail injection system, that is to say the pressure prevailing in the fuel between the high-pressure pump and the injection valve. For that reason the system typically has a rail pressure sensor in the region of the common feed, referred to as the common rail, between the high-pressure pump and the injectors in order to enable the rail pressure to be determined.
Accordingly, if the pressure sensor is defective, there is no information concerning the rail pressure available to the engine management system, with the result that it becomes impossible to control the injection quantity. Furthermore, a number of different faults can occur which lead to the rail pressure in the common rail system rising to a value that lies above the measurement range of the pressure sensor. This can be the case, for example, if a flow control valve of the pump sticks in an open position. In this case too there is a lack of the pressure information that is necessary for controlling the injection quantity.